The chief of staff to Jean-Claude Juncker is notorious for his fanatical devotion to the ideology of European unification

He has managed to gain another nickname - the “Rasputin of Brussels” – but doesn’t seem phased by it, telling the FT: “If you look into the history of Rasputin, that can be both flattering and not — Lenin can be flattering or not.

“If it means there is an efficient manager, somebody who is not a wimp, I’m OK with that.

“You can’t run the European Commission like a Montessori school.”

And unlike other civil servants, who choose to remain out of the spotlight, he has developed a persona of his own, memorably tweeting in May last year that a G7 meeting with “Trump, Le Pen, Boris Johnson and Beppe Grillo” would be “a horror scenario that shows well why it is worth fighting populism”.

Juncker dispels reports claiming Prime Minster Theresa May ‘begged him for help’ at a Brussels dinner

But he hit the headlines for all the wrong reasons, after being accused of leaking a hugely damaging account of a Downing Street dinner between Theresa May, his boss and the EU’s chief Brexit negotiator Michel Barnier.

Headlined “The Disastrous Brexit Dinner”, it was described back in April as am unparalleled act of indiscretion aimed at derailing the negotiations.

It said that after the meal in No10 Mr Juncker picked up the phone to German chancellor Angela Merkel and declared that the Prime Minister was in “a different galaxy”.

It was also reported that Mr Juncker remarked to aides: “I’m leaving Downing Street 10 times more sceptical than I was before.”

He has managed to gain the nickname the 'Rasputin of Brussels' due to his authoritarian style

Mrs May’s former chief of staff Nick Timothy accused Mr Selmayr of being the source of the leak – which said she “begged” for help in moving the negotiations forward.

He tweeted: “After constructive Council meeting, Selmayr does this. Reminder that some in Brussels want no deal or a punitive one.”

But the German aide denied he was involved – suggesting he had been framed due to his reputation.

He replied on social media: “This is false. I know it doesn’t fit your cliché, Mr Timothy. But Mr Juncker and I have no interest in weakening [the] PM.

Theresa May and Jean-Claude Juncker agree Brexit negotiations should 'accelerate over the months to come

“But it seems some have interest in undermining constructive relations [between] Mr Juncker and PM May. Who is the real question.”

He added: “I deny that 1) we leaked this; 2) Mr Juncker ever said this; 3) we are punitive on Brexit. It's an attempt to frame EU side and to undermine talks.”

It comes after it was reported Mr Juncker made a point of saying as Mr May and the Brexit Secretary David Davis were leaving the dining room at the Berlaymont – while they were still in earshot: “Now we won’t have any leaks of this will we Martin?”

A trained lawyer, Mr Selmayr studied at the University of Passau, at the University of Geneva and at King’s College London.

It was during his time studying in London his antipathy towards Britain may have been cemented after he went to hear Margaret Thatcher speak

On the other hand his commitment to the European project stems from a trip to the battlefields and military cemeteries of Verdun as a teenager with his maternal grandfather, Heinz Gaedecke.

He told Politico: “To protect the peace, to protect prosperity, to protect fundamental freedoms — that, I think, is a reason to get up every morning,”

But his family on his father’s side reveals a darker past, as his paternal grandfather Josef Selmayr was convicted of war crimes after serving a Nazi Lieutenant Colonel on Hitler's general staff in the Balkans during World War Two.

He won three bravery awards including the Iron Cross and was sentenced to 15 years in prison. After serving five he was released and later helped to create West German military intelligence.